1. Technical Field
The present disclosure relates to a semiconductor device and to a method of forming the same and, and more particularly, to a semiconductor device with a high breakdown voltage and to a method of forming the same.
2. Description of the Related Art
A cell array of a mask read only memory (ROM) in semiconductor memory devices may include an on-transistor and an off-transistor. In forming the off-transistor of the cell array, transistors with a low threshold voltage Vth are typically formed on an entire array region. Next, a dopant (e.g., p-type dopant, boron) of a conductive type, which is opposite to that of a source region and a drain region of the on-transistor, is selectively doped on a channel region of the transistor using a masking process. The above-mentioned masking process results in the transistor becoming an off-transistor with a high threshold voltage. Moreover, predetermined data is formed using the on-transistor and the off-transistor during the semiconductor device manufacturing processes.
FIG. 1 is a graph illustrating a change of a well density and a transistor breakdown voltage BVdss in an on-transistor and an off-transistor according to a design rule of a conventional semiconductor device. In the above off-transistor, when the size of a semiconductor device is reduced, the dopant concentration of the channel region in the semiconductor device should be increased to obtain an appropriate threshold voltage. Moreover, the junction breakdown voltage may be reduced according to a gradually increasing density in a well and a semiconductor substrate, on which the semiconductor devices are formed. Additionally, when an additional ion injection is performed to form the off-transistor with a higher threshold voltage, the breakdown voltage may be even further reduced and the leakage current may be increased.
Thus, there is a need for a semiconductor device with a high breakdown voltage and to a method of forming the same.